2 cyl's quit after about 10 minutes of riding

[Anonymous]

I have a 79 gs750 that I bought 2 summers ago. Cheap. I got it home and found that I had a bad coil on it... I've replaced the coils with new dyna coils. I replaced the ign with a new dyna II ign, and I replaced the wires on it. Well it will run like a CHAMP for the 1st 5-10 minutes, but when I get on a hgwy and get r goin at high rpm she wont last long. So I took it to a friend of my pops who owns a bike shop and works on older bikes. I thought it was a carb problem, so thats why I took it to him. Last time we talked (last summer) he said he would ride it and it would run fine but soon as it got hot enough cyl 3 and 4 wouldnt fire so he went through the carbs and the problem still exists, he still has the bike. I called him up earlier this week, and now he thinks its the ignition... I know its 1-4 and 2-3, but he said to try to find the old points system and bring it out (which is over an hour away) to try to eliminate the ign problem, and I cant find the old points plate or points and condensers, I think I tossed them. I'm just wondering if anybody has run across this problem with their dyna elec ignition... I'll be damned if I go through another summer without riding it. I've only put maybe 30 miles on this thing since I got it like 2 yrs ago!! Anybody have any ideas!!??

[Anonymous]

it could be a dyna III ign... It has the seperate ign box

[Anonymous]

750

You dont have to do all that, all you have to do is run it till it quits and then check it for spark,(with a spare plug)and if it does then, when it quits next time pop open the gas cap and see if vacuum escapes when you open it up, if so you have a plugged tank vent

[dpep]

I may have your answer. If I do it is a real cheap fix. It is certainly worth checking out.

http://www.thegsresources.com/_forum/viewtopic.php?p=299964#299964

[KEITH KRAUSE]

Doesn't sound like coil or ignition modules to me, because your problems are on 3 and 4, which are seperate coils/modules.
Are the plugs black at 3 and 4?
Any leaking out of the 3 and 4 float bowl overflow lines underneath?
Possible simplist fixes:if the pilot screw (underneath and engine side of the bowl) has been turned out too much, you'll foul the plug. Turn the pilot screw out on 3 and 4 one full turn from LIGHTLY seated for starters. Use a good fitting tool if they're stiff.
Then adjust the side air screws out to 1 1/2 turns from lightly seated.
Then SYNCH the carbs with a vacuum tool. Your VM carbs are very sensitive to vacuum imbalances between cylinders. If you don't have a vacuum tool, then at least try the screw adjustments. Keep a record of where your screws are now so you can return them if you don't like the results. Motion Pro makes a very simple and cheap vacuum tool for about $30/35. Your problem could definitely be a poor synch.
If that doesn't stop the problem, then the carbs should be cleaned, o-rings inspected/replaced, and the float heigths adjusted and the float needle valves inspected/replaced. If you remove the carbs, I recommend replacing the manifold o-rings too.
There's always the possibility that someone changed the jetting too, but didn't do all the carbs the same way for whatever reason. As a bike warms up, it runs richer. If your having richness problems, it will effect the bike more as it warms up and could be causing your problem.

[Anonymous]

#34

Your right Keith I had to go back and reread it, I wonder if he put the old plug caps back after changeing the coils